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Old 02-15-2008, 01:29 PM   #1 (permalink)
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Default PO325 Knock Sensor

Please, oh please someone who knows FOR SURE... Just exactly what are ALL the conditions that will set a PO325 code in a '97 OB ECM.

I wish Subaru would make public the exact algorithms used for the various codes. In some case they are obvious, in others (EGR low flow for instance) they are NOT.
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Old 02-15-2008, 02:19 PM   #2 (permalink)
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Hi. Check your knock sensor, it may have cracked or have a bad connection. That is not uncommon on soobs of that era. Generally replacing the knock sensor takes care of it. Of course it can be other issues such as a wiring issue back to the ECM, etc. It is an electrical issue.

The troubleshooting procedure for all codes is in the Subaru service manual for that vehicle.

DTC PO325: KNOCK SENSOR CIRCUIT MALFUNCTION (KNOCK)
DTC DETECTING CONDITION: Immediately at fault recognition
TROUBLE SYMPTOM: Poor driving performance, Knocking occurs.


Also make sure the connection betweent the knock sensor and block is clean.


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PO325 Knock Sensor-79056096.gif  
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Old 02-15-2008, 05:33 PM   #3 (permalink)
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I just replaced mine a couple of weeks ago. It was cracked just like the picture. $59.00 at subarupartsforyou.com. and a 10 minute job to complete.
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Old 02-15-2008, 05:49 PM   #4 (permalink)
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Thanks Guys,

I am quite familiar with Subarus as I was a Used Parts Manager for an used car dealer... I would buy wrecks and part them out and sell.

My problem is the sensor WAS replaced with a new Subie unit I HAVE verified the wiring continuity and ensure no short to ground exists on either the signal lead OR the shield (ECM plug pins 3 and 56). No, the code comes right back upon being cleared (engine running at idle). I even did the unthinkable "Easter Egging" of swapping out the ECM with a known good unit. NO GO.

There's something rotten in Denmark and I REALLY NEED to know what the ECM is looking for to set the code. PLUS, to make matters worse, the scan tool shows spark advancing/retarding form 15 to 39 deg with throttle blipping (as expected). SO the PO325 condition is not permanantly retarding the ignition. Wassup?
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Old 02-15-2008, 10:52 PM   #5 (permalink)
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hehe well you didn't mention all that at first! So have you followed the troubleshooting procedure in the factory service manual with all its resistances voltages etc to check? Do you have any other/additional codes or just P0325?

Here's a quick test - put say a ~500 kiloohm resistor from the knock sensor connector to ground as a test (in place of the knock sensor) and see if you still ge the code.
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Old 02-16-2008, 08:52 AM   #6 (permalink)
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Was the new sensor properly installed? Was the mating surface on the engine block cleaned ? Was the bolt properly torqued, and the wire properly routed? If not, it will throw the code again. If the bolt wasn't torqued correctly, it could be loose... I would check these first.
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Old 02-16-2008, 04:52 PM   #7 (permalink)
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I was using Mitchell's S/W (provided by the owner of the used car dealership I am consulting to). The "Troubleshooting" methodology basically is make sure there is electrical continuity from sensor to the ECM (pins as specified in previous post). Nothing below 700 K ohms, etc. As I said electrically everything is kosher. What I want to know is just WHAT CONDITIONS the ECM looks for to set the code. For instance...If I substitute a 1/2 meg resistance for the sensor then obviously there will be no AC signal (other than some small amount of noise) to the ECM with engine running... Is this a condition that will cause the ECM to declare a PO325?
How about some kind of test to verify operability when there is no short ckt present but, say, a broken lead inside the sensor to the crystal itself. It would seem to me that this situation is functionally the same as the 500 Kohm resistor scenerio. Is there some algorithm that advances the ignition to get a "knock" for a couple crank revolutions and then backs off when the sensor "hears" it? Or is that too sophisticated for a simple ECM? If I hook a lab scope up to the backprobed ECM what is a "normal" background signal?

I am reminded of the problems I encountered during the swap out of a non-EGR 2.2 to replace a twin-cam 2.5. (something I have now done some 15 times at the dealership I used to work for as Used Parts Manager). It seems that the ECM checked for open open CKT of the EGR solenoid valve AND also looked for front O2 sensor temperature change when the solenoid called for opening of the EGR valve itself...thus negating "fooling" the ECM by just substituting a resistor for a non-existant EGR solenoid. So you see I am more than a little curious about the knock sensor situation.
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Old 02-16-2008, 05:16 PM   #8 (permalink)
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I can do some research later tonight, but AFIAK the ECU does not care if it does not see the ac signal but it wants to see roughly the right resistance in the circuit (around 500kohm). On my '00obw I removed the knock sensor and put a 470kohm resistor in its place. With the sensor unhooked I got a CEL. With the resistor in place I have never gotten a CEL and the timing appears to advance as far as the map goes.

On endwrench.com there is a description of how the knock sensor circuit works. I can find it later. Basically if the ECU 'hears' what it thinks is knock, it starts retarding timing until the knock goes away, then advances again.
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Old 02-16-2008, 05:27 PM   #9 (permalink)
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Thanks for your reply. It was interesting to hear of your empirical observations and I just may do the same to fix this poor bas****'s MIL situation.

BUT if you have the time to give me a few links to look at or just some more specifics I would be more than willing to share my knowledge of Subarus to the extent I can. Ferinstance is your '00 OB an automatic? If so do you have a 'slow going into gear' situation (or have you ever experienced this?) I found exactly what causes this and (if you are moderately capable) how to correct it for about $50 of Subaru seals and a few hours work.
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Old 02-16-2008, 09:20 PM   #10 (permalink)
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Excellent. Yes my info on the knock sensor is anecdotal. I got the resistor tip from some other threads on a different board from people doing the same thing.

Ok...links let's see
Here's a link, might not be real helpful to your situation: http://endwrench.com/images/pdfs/ModelsInfo.pdf
Models Equipped With Knock Sensors
The knock sensor is designed to sense knocking signals from each cylinder. The
knock sensor is a piezo-electric type element which converts knocking vibrations into electrical signals. The electrical signal is sent to the ECM, which changes the ignition timing to reduce the engine knock or ping. For this system to work correctly, the sensor must first hear the engine ping.

The driver of the vehicle may also hear a small engine ping. A delay of approximately 1-2 seconds is normal, depending on the fuel quality, engine load, air temp, etc. At this time, the ECM will retard the timing. This function can be viewed on the Select Monitor RTRD mode. When the knock is eliminated, the timing is gradually advanced to the specified setting. If engine ping is heard again this process is repeated.

I did find this info about P0325 in a www.endwrench.com article...though it says for '95 it could apply to other vehicles?
1995 Subaru Legacy - DTC PO325 - Knock Sensor Circuit Malfunction
If you encounter a Check Engine Light with a DTC P0325 on a 1995 Subaru Legacy, check to see if the knock sensor mounting bolt has been torqued too tightly upon installation.

Also check the threads of the sensor mounting bolt and the thread surfaces of the block to see that they are clean and not corroded. Any corrosion should be removed before installing the bolt. The correct mounting bolt torque is 15-19 ft. lbs.



My '00obw fortunately so far has not had the slow to engage into drive issue, though yes I have seen lots of posts with this issue on '99 and '00 legacy/outback. Is it this part #? There are a couple threads on here from people who had this isue: -> Transmission: Wont go !!

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